Surface cleaning method of semiconductor wafer heat treatment boat

ABSTRACT

A surface cleaning method of a semiconductor wafer heat treatment boat that can prevent metallic contamination to semiconductor wafers and keep down a production time and manufacturing costs of semiconductor wafers by efficiently and easily removing metallic impurities in an oxide film on an SiC boat surface is provided. A surface cleaning method of a semiconductor wafer heat treatment boat according to an embodiment of the present invention is a surface cleaning method of a semiconductor wafer heat treatment boat whose surface is formed of SiC, includes oxidizing the surface of the heat treatment boat by thermal oxidation and etching a portion of the oxide film formed after oxidation is removed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2006-262305, filed on Sep. 27, 2006, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a surface cleaning method of a semiconductor wafer heat treatment boat, and in particular, relates to a surface cleaning method of a semiconductor wafer heat treatment boat whose surface is made of SiC.

BACKGROUND OF THE INVENTION

Semiconductor devices such as LSI are formed on the surface of a semiconductor wafer (hereinafter simply called a “wafer”). In a manufacturing process of the semiconductor wafer, a process of heat-treating the wafer at a high temperature is performed in order to form an oxide film on the wafer surface or diffuse impurities.

In such a process of providing heat treatment, a component for holding the wafer during heat treatment, a so-called semiconductor wafer heat treatment boat (hereinafter called a “boat”) is needed. SiO₂ (quarts), Si (silicon), SiC (silicon carbide) and the like have been used as material for the semiconductor wafer heat treatment boat.

Then, in recent years, a semiconductor wafer heat treatment boat obtained by coating the surface of a boat formed of Si-impregnated SiC base material with CVD-SiC coating has been mainstream (for example, JP-A 2000-119079 (KOKAI)). Such a semiconductor wafer heat treatment boat has advantages that heat resistance is high, largely-shaped products are relatively easy to mold, and contamination from the boat to the wafer can be reduced by covering the surface thereof with CVD-SiC, which can easily be highly purified.

However, if a wafer is heat-treated using a semiconductor wafer heat treatment boat whose surface is coated with CVD-SiC, free carbon (C) in CVD-SiC will react with the wafer. Then, particulate SiC will attach to the wafer surface as a reaction product. Such a reaction product is unpreferred because it degrades surface smoothness of the wafer and leads to a lower yield of semiconductor devices formed later on the wafer.

Thus, in order to suppress attachment of reaction products onto the wafer, an oxide film of about 100 nm has conventionally been formed on the surface by thermally oxidizing the boat. However, though reaction products are suppressed, formation of the oxide film has created a new problem that metallic impurities such as Fe supplied by, for example, a source gas of oxidization are incorporated into the oxide film to cause metallic contamination for the wafer. Then, to solve this problem, a method of out-diffusion to diffuse metallic impurities such as Fe into an inert atmosphere by heat-treating a boat in the inert atmosphere at a high temperature for a long time has generally been applied.

Since the oxide film formed on the SiC surface of the boat is etched each time the heat treatment process of the wafer is performed, the thickness of the film will indeed be thinner. Thus, periodic additional oxidization treatment is needed, but there arises each time a need to add high-temperature heat treatment to cause metallic contamination to diffuse outwardly. Therefore, this has led to an increased production time of semiconductor wafer and increased manufacturing costs.

Accordingly, a surface cleaning method that can efficiently and easily remove metallic contamination of a semiconductor wafer heat treatment boat whose surface is made of SiC has been demanded.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances and an object thereof is to provide a surface cleaning method of a semiconductor wafer heat treatment boat that can prevent metallic contamination to semiconductor wafers and keep down the production time and manufacturing costs of semiconductor wafers by efficiently and easily removing metallic contamination in an oxide film on the SiC boat surface.

The surface cleaning method of a semiconductor wafer heat treatment boat according to an aspect of the present invention is a surface cleaning method of a semiconductor wafer heat treatment boat whose surface is formed of silicon carbide (SiC), including oxidizing the surface of the semiconductor wafer heat treatment boat by thermal oxidation and etching a portion of an oxide film formed after oxidation of the surface is etched.

According to an embodiment of the present invention, metallic contamination to semiconductor wafers is prevented by efficiently and easily removing metallic contamination in an oxide film on the SiC boat surface and, as a result, a surface cleaning method of a semiconductor wafer heat treatment boat that can keep down the production time and manufacturing costs of semiconductor wafers can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a process flow of a surface cleaning method of a semiconductor wafer heat treatment boat in an embodiment;

FIG. 2 is a diagram showing measurement results by SIMS (Secondary Ion Mass Spectrometry) of Fe concentrations in an oxide film formed on an SiC boat surface in the embodiment;

FIG. 3 is a diagram showing measurement results by SIMS (Secondary Ion Mass Spectrometry) of Fe concentrations in the oxide film formed on the SiC boat surface in the embodiment;

FIG. 4 is a diagram showing measurement results by SIMS (Secondary Ion Mass Spectrometry) of Fe concentrations in an oxide film formed on the SiC boat surface in the embodiment;

FIG. 5 is a diagram showing an average Fe concentration at a wafer support position by a boat in an example; and

FIG. 6 is a diagram showing average Fe concentrations at the wafer support position by the boat when the example is repeatedly heat-treated.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have found that when the SiC boat surface is oxidized, metallic impurities contained in an oxide film, particularly Fe is unevenly distributed on the surface of the oxide film and, based on this finding, have completed the present invention. An embodiment of the surface cleaning method of a semiconductor wafer heat treatment boat (hereinafter also simply called a boat) in regard to the present invention will be described below with reference to attached drawings.

First, FIG. 2 shows measurement results by SIMS (Secondary Ion Mass Spectrometry) of Fe concentrations in an oxide film formed on the SiC boat surface. The SiC boat is a boat obtained by coating the surface of a boat formed from Si-impregnated SiC base material with CVD-SiC with a thickness of about 60 μm. CVD-SiC was oxidized by dry oxidation for one hour at 1100° C. and 1200° C. to form oxide films of 61 nm and 270 nm respectively.

As is evident from FIG. 2, the Fe concentration is highest at the top surface of the oxide film in both temperature conditions of 1100° C. and 1200° C. Then, after decreasing rapidly from the surface to a depth of about 10 nm, there is a tendency to decrease gradually.

The surface cleaning method of a semiconductor wafer heat treatment boat in the present embodiment is a surface cleaning method of a semiconductor wafer heat treatment boat whose surface is formed of silicon carbide (SiC). Then, the surface of the semiconductor wafer heat treatment boat is oxidized by thermal oxidation and a portion (not a whole) of an oxide film formed after oxidation of the surface is etched.

Then, when etching the portion of the oxide film, an HF solution is used as an etchant. Moreover, the semiconductor wafer heat treatment boat in the present embodiment is a silicon wafer heat treatment boat.

FIG. 1 is a schematic sectional view showing a process flow of a surface cleaning method of a semiconductor wafer heat treatment boat in an embodiment. A sectional view in FIG. 1 is a sectional view cutting out a portion of the boat including the boat surface.

FIG. 1A is a portion of a semiconductor wafer heat treatment boat formed of an Si-impregnated SiC base material 100 with CVD-SiC 102 deposited on the surface thereof.

Next, as shown in FIG. 1B, the CVD-SiC 102 of the boat shown in FIG. 1A is thermally oxidized at a temperature of about 1100° C. and 1200° C. to form an oxide film 104 of about 60 nm to 300 nm in thickness on the CVD-SiC surface. This oxide film 104 prevents generation of SiC deposits on a wafer when the wafer is heat-treated later. At this point, as shown in FIG. 1B, an Fe-contaminated layer 120 of the oxide film containing a high concentration of Fe incorporated from an oxidizing atmosphere exists near the surface of the oxide film 104.

Next, as shown in FIG. 1C, the Fe-contaminated layer 120 on the boat surface is removed by etching the boat in a diluted HF solution of about 0.1% to 1.0% for 2 min to 15 min. At this point, a portion of the oxide film 104 is always made to remain.

Metallic contamination on the boat has conventionally been removed by providing high-temperature heat treatment for several hours in an inert atmosphere at a temperature of about 1200° C., as described above, to diffuse metallic contamination such as Fe outwardly into the inert atmosphere. Then, by removing metallic contamination on the boat surface, metallic contamination to wafers to be treated later using the boat has been prevented. Indeed, heat treatment of out-diffusion at a high temperature for a long time is needed each time an oxide film is formed on the boat, entailing enormous costs.

In contrast, according to the present embodiment, removal of metallic contamination can be realized by wet etching of 2 min to 15 min. Thus, a time needed for surface cleaning treatment of a boat can greatly be reduced and therefore, significant cost reduction can be realized.

Incidentally, it is desirable that the treatment temperature for forming a thermal oxide film on the boat surface is 1100° C. or more and 1200° C. or less. This is because, if the temperature is below this range, the oxidation time required to acquire a necessary oxide film will be long, generating a concern of rising costs. If, on the other hand, the temperature is above this range, the concentration of Fe incorporated into the contaminated layer will be higher, causing the cleaning efficiency of a boat according to the present embodiment to degrade.

Also, it is desirable that the amount of etching by a diluted HF solution in the process of removing the Fe-contaminated layer 120 on the boat surface is 10 nm or more, and desirably 20 nm or more. As shown in FIG. 3, a high concentration of Fe exists up to the depth of about 10 nm from the surface. For oxidation at 1200° C., it is estimated from SIMS profiles that a somewhat high concentration of Fe still exists further deeper. For oxidation at 1100° C., a high concentration of Fe exists up to the depth of about 10 nm and the concentration will be lower further deeper. Thus, for oxidation at 1200° C. and oxidation at 1100° C., Fe contamination to wafers can adequately be reduced by etching of 10 nm or more and, for oxidation at 1200° C., Fe contamination to wafers can further be reduced by etching of 20 nm or more.

FIG. 4 shows results of SIMS analysis of 24 nm etching in a diluted HF solution after forming an oxide film of 61 nm in thickness on the surface of CVD-SiC under conditions of dry oxidation at 1100° C. for one hour. Closed circles denote Fe concentrations before HF etching and open circles denote those after HF etching. As is evident from FIG. 4, while the Fe concentration at the top surface before HF etching was about 1.8E17 atoms/cm³, the Fe concentration after HF etching was about 9E15 atoms/cm³, a reduction in Fe concentration on the surface of more than an order of magnitude. This result also shows that, according to the present embodiment, metallic contamination such as Fe on the boat surface can efficiently and easily be removed.

Incidentally, when etching a portion of an oxide film to remove the Fe-contaminated layer on the boat surface, it is desirable to keep the amount of etching of the oxide film to 50% or less of the thickness of the oxide film after oxidation. This is because, if etched further, a loss portion (unused portion) of the formed oxide film will be larger, leading to inefficient surface cleaning treatment.

The embodiment of the present invention has been described above with reference to concrete examples. Though only a semiconductor heat treatment boat and a surface cleaning method of a semiconductor heat treatment boat have been described in the description of the embodiment and what is not directly needed for the description of the present invention is omitted, elements relating to the required semiconductor heat treatment boat and the surface cleaning method of a semiconductor heat treatment boat can suitably be selected and used.

In addition, all surface cleaning methods of a semiconductor heat treatment boat that can suitably be designed or modified by a person skilled in the art are included in the scope of the present invention.

For example, the present embodiment has been described by taking a semiconductor wafer heat treatment boat formed from Si-impregnated SiC base material with CVD-SiC deposited on the surface thereof as an example. However, the present invention can also be applied if the boat of SiC base material is not impregnated with Si.

Also, an HF solution was used as an etchant in the process of removing a contaminated layer, any etchant that can etch an oxide film, for example, ammonium fluoride or other etchants may be applied. Further, any process that can etch an oxide film, for example, dry etching, may be used.

EXAMPLE

Examples of the present invention will be described below, but the present invention is not limited to these examples.

A semiconductor wafer heat treatment boat for semiconductor wafers of 300 mm in diameter formed from Si-impregnated SiC base material with CVD-SiC deposited on the surface thereof was dry-oxidized at 1200° C. for one hour. An oxide film of about 270 nm was formed on the boat surface by the oxidation. The boat was soaked in a 0.5% HF solution for 2 min to remove the oxide film of up to about 10 nm depth from the surface of the oxide film by etching.

50 silicon wafers were loaded on the boat that underwent the above surface cleaning treatment to heat-treat the silicon wafers in an argon gas atmosphere at 1200° C. for one hour. After the heat treatment, the average Fe concentration of three silicon wafers at the wafer support position by the boat was measured by the SPV (Surface Photo Voltage) method. FIG. 5 shows a measurement result.

The condition of the argon gas atmosphere at 1200° C. is generally a heat treatment condition often used for conventional out-diffusion heat treatment.

Then, a similar experiment of a boat for which no etching by the HF solution had been performed was conducted as a comparative example. In addition, as the above comparative example, experiments of further providing heat treatment in the argon gas atmosphere at 1200° C. for one hour by loading silicon wafers on a boat, which underwent no etching by the HF solution and underwent heat treatment once or a plurality of times in the argon gas atmosphere at 1200° C. for one hour, were conducted. Then, after each heat treatment, the average Fe concentration of silicon wafers at the wafer support position by the boat was measured by the SPV (Surface Photo Voltage) method to monitor for change. FIG. 6 shows measurement results.

First, FIG. 5 shows the average Fe concentration of the silicon wafers at the wafer support position by the boat after the first heat treatment at 1200° C. for one hour. The Fe concentration for the comparative example, that is, when no etching by the HF solution is performed is represented as 100%. As is evident from FIG. 5, the Fe concentration of the example is ⅕ of that of the comparative example. Therefore, the example confirms that Fe contamination of wafers is effectively reduced.

Next, FIG. 6 shows results of Fe concentration measurement after providing heat treatment by loading wafers of the comparative example on the boat, which underwent heat treatment once or a plurality of times under conditions of the argon gas atmosphere at 1200° C. similar to those used for out-diffusion heat treatment. That is, the heat treatment count three times on the horizontal axis in FIG. 6 indicates that silicon wafers are loaded on a boat that twice underwent heat treatment in the argon gas atmosphere at 1200° C. for one hour to further undergo heat treatment in the argon gas atmosphere at 1200° C. for one hour.

As is evident from FIG. 6, when the heat treatment count is six times, a result equivalent to the Fe concentration of the example is obtained for the comparative example. In other words, it turned out that, by applying the present invention, an Fe-contaminated layer removal effect comparable to a case in which high-temperature heat treatment, which is a conventional Fe-contaminated layer removal method, is performed in the argon gas atmosphere at 1200° C. for five hours can be gained. In view of the fact that the etching time is two minutes in the example, the example confirms that, by applying the present invention, the time required for surface cleaning of a semiconductor wafer heat treatment boat can greatly be reduced. 

1. A surface cleaning method of a semiconductor wafer heat treatment boat whose surface is made of silicon carbide (SiC), comprising; Oxidizing the surface of the semiconductor wafer heat treatment boat by thermal oxidation, and Etching a portion of an oxide film formed after oxidation of the surface.
 2. The method according to claim 1, wherein an HF solution is used as an etchant for etching the portion of the oxide film.
 3. The method according to claim 1, wherein 10 nm or more of the oxide film is etched when the portion of the oxide film is etched.
 4. The method according to claim 1, wherein a treatment temperature of the thermal oxidation is 1100° C. or more and 1200° C. or less.
 5. The method according to claim 1, wherein the oxide film has a thickness of 60 nm or more and 300 nm or less.
 6. The method according to claim 1, wherein 50% or less of a thickness of the oxide film is etched when the portion of the oxide film is etched.
 7. The method according to claim 1, wherein the boat is formed of an Si-impregnated SiC base material with CVD-SiC deposited on the surface thereof.
 8. The method according to claim 1, wherein the semiconductor wafer is a silicon wafer. 